diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index 89ab4027ed8..e10c4ed8a8c 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -31,15 +31,16 @@ namespace ts {
     }
 
     export function getDeclarationDiagnostics(host: EmitHost, resolver: EmitResolver, targetSourceFile: SourceFile): Diagnostic[] {
-        let diagnostics: Diagnostic[] = [];
-        let { declarationFilePath } = getEmitFileNames(targetSourceFile, host);
-        if (declarationFilePath) {
-            emitDeclarations(host, resolver, diagnostics, declarationFilePath, targetSourceFile);
+        let declarationDiagnostics = createDiagnosticCollection();
+        forEachExpectedEmitFile(host, getDeclarationDiagnosticsFromFile, targetSourceFile);
+        return declarationDiagnostics.getDiagnostics(targetSourceFile.fileName);
+
+        function getDeclarationDiagnosticsFromFile({ declarationFilePath }, sources: SourceFile[], isBundledEmit: boolean) {
+            emitDeclarations(host, resolver, declarationDiagnostics, declarationFilePath, !isBundledEmit ? targetSourceFile : undefined);
         }
-        return diagnostics;
     }
 
-    function emitDeclarations(host: EmitHost, resolver: EmitResolver, diagnostics: Diagnostic[], declarationFilePath: string, root?: SourceFile): DeclarationEmit {
+    function emitDeclarations(host: EmitHost, resolver: EmitResolver, emitterDiagnostics: DiagnosticCollection, declarationFilePath: string, root?: SourceFile): DeclarationEmit {
         let newLine = host.getNewLine();
         let compilerOptions = host.getCompilerOptions();
 
@@ -243,14 +244,14 @@ namespace ts {
                 let errorInfo = writer.getSymbolAccessibilityDiagnostic(symbolAccesibilityResult);
                 if (errorInfo) {
                     if (errorInfo.typeName) {
-                        diagnostics.push(createDiagnosticForNode(symbolAccesibilityResult.errorNode || errorInfo.errorNode,
+                        emitterDiagnostics.add(createDiagnosticForNode(symbolAccesibilityResult.errorNode || errorInfo.errorNode,
                             errorInfo.diagnosticMessage,
                             getSourceTextOfNodeFromSourceFile(currentSourceFile, errorInfo.typeName),
                             symbolAccesibilityResult.errorSymbolName,
                             symbolAccesibilityResult.errorModuleName));
                     }
                     else {
-                        diagnostics.push(createDiagnosticForNode(symbolAccesibilityResult.errorNode || errorInfo.errorNode,
+                        emitterDiagnostics.add(createDiagnosticForNode(symbolAccesibilityResult.errorNode || errorInfo.errorNode,
                             errorInfo.diagnosticMessage,
                             symbolAccesibilityResult.errorSymbolName,
                             symbolAccesibilityResult.errorModuleName));
@@ -266,7 +267,7 @@ namespace ts {
         function reportInaccessibleThisError() {
             if (errorNameNode) {
                 reportedDeclarationError = true;
-                diagnostics.push(createDiagnosticForNode(errorNameNode, Diagnostics.The_inferred_type_of_0_references_an_inaccessible_this_type_A_type_annotation_is_necessary,
+                emitterDiagnostics.add(createDiagnosticForNode(errorNameNode, Diagnostics.The_inferred_type_of_0_references_an_inaccessible_this_type_A_type_annotation_is_necessary,
                     declarationNameToString(errorNameNode)));
             }
         }
@@ -1616,14 +1617,14 @@ namespace ts {
     }
 
     /* @internal */
-    export function writeDeclarationFile(declarationFilePath: string, sourceFile: SourceFile, host: EmitHost, resolver: EmitResolver, diagnostics: Diagnostic[]) {
-        let emitDeclarationResult = emitDeclarations(host, resolver, diagnostics, declarationFilePath, sourceFile);
+    export function writeDeclarationFile(declarationFilePath: string, sourceFile: SourceFile, host: EmitHost, resolver: EmitResolver, emitterDiagnostics: DiagnosticCollection) {
+        let emitDeclarationResult = emitDeclarations(host, resolver, emitterDiagnostics, declarationFilePath, sourceFile);
         let emitSkipped = emitDeclarationResult.reportedDeclarationError || host.isEmitBlocked(declarationFilePath);
         if (!emitSkipped) {
             let declarationOutput = emitDeclarationResult.referencePathsOutput
                 + getDeclarationOutput(emitDeclarationResult.synchronousDeclarationOutput, emitDeclarationResult.moduleElementDeclarationEmitInfo);
             let compilerOptions = host.getCompilerOptions();
-            writeFile(host, diagnostics, declarationFilePath, declarationOutput, compilerOptions.emitBOM);
+            writeFile(host, emitterDiagnostics, declarationFilePath, declarationOutput, compilerOptions.emitBOM);
         }
         return emitSkipped;
 
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 0be2549b969..b9dad7e151e 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -322,18 +322,15 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promi
         let languageVersion = compilerOptions.target || ScriptTarget.ES3;
         let modulekind = compilerOptions.module ? compilerOptions.module : languageVersion === ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.None;
         let sourceMapDataList: SourceMapData[] = compilerOptions.sourceMap || compilerOptions.inlineSourceMap ? [] : undefined;
-        let diagnostics: Diagnostic[] = [];
+        let emitterDiagnostics = createDiagnosticCollection();
         let emitSkipped = false;
         let newLine = host.getNewLine();
 
         forEachExpectedEmitFile(host, emitFile, targetSourceFile);
 
-        // Sort and make the unique list of diagnostics
-        diagnostics = sortAndDeduplicateDiagnostics(diagnostics);
-
         return {
             emitSkipped,
-            diagnostics,
+            diagnostics: emitterDiagnostics.getDiagnostics(),
             sourceMaps: sourceMapDataList
         };
 
@@ -949,7 +946,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promi
                     }
                     else {
                         // Write source map file
-                        writeFile(host, diagnostics, sourceMapData.sourceMapFilePath, sourceMapText, /*writeByteOrderMark*/ false);
+                        writeFile(host, emitterDiagnostics, sourceMapData.sourceMapFilePath, sourceMapText, /*writeByteOrderMark*/ false);
                     }
                     sourceMapUrl = `//# sourceMappingURL=${sourceMapData.jsSourceMappingURL}`;
 
@@ -1037,7 +1034,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promi
             }
 
             function writeJavaScriptFile(emitOutput: string, writeByteOrderMark: boolean) {
-                writeFile(host, diagnostics, jsFilePath, emitOutput, writeByteOrderMark);
+                writeFile(host, emitterDiagnostics, jsFilePath, emitOutput, writeByteOrderMark);
             }
 
             // Create a temporary variable with a unique unused name.
@@ -8134,7 +8131,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promi
             }
 
             if (declarationFilePath) {
-                emitSkipped = writeDeclarationFile(declarationFilePath, isBundledEmit ? undefined : sourceFiles[0], host, resolver, diagnostics) || emitSkipped;
+                emitSkipped = writeDeclarationFile(declarationFilePath, isBundledEmit ? undefined : sourceFiles[0], host, resolver, emitterDiagnostics) || emitSkipped;
             }
         }
     }
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 0b22657a694..fea71cfc67e 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -1883,9 +1883,9 @@ namespace ts {
         return combinePaths(newDirPath, sourceFilePath);
     }
 
-    export function writeFile(host: EmitHost, diagnostics: Diagnostic[], fileName: string, data: string, writeByteOrderMark: boolean) {
+    export function writeFile(host: EmitHost, diagnostics: DiagnosticCollection, fileName: string, data: string, writeByteOrderMark: boolean) {
         host.writeFile(fileName, data, writeByteOrderMark, hostErrorMessage => {
-            diagnostics.push(createCompilerDiagnostic(Diagnostics.Could_not_write_file_0_Colon_1, fileName, hostErrorMessage));
+            diagnostics.add(createCompilerDiagnostic(Diagnostics.Could_not_write_file_0_Colon_1, fileName, hostErrorMessage));
         });
     }