Treat multiple prototype property assignments as union property declarations

src/compiler/checker.ts

@@ -2827,7 +2827,7 @@ namespace ts {
                 }
                 // Handle module.exports = expr
                 if (declaration.kind === SyntaxKind.BinaryExpression) {
-                    return links.type = checkExpression((<BinaryExpression>declaration).right);
+                    return links.type = getUnionType(map(symbol.declarations, (decl: BinaryExpression) => checkExpressionCached(decl.right)));
                 }
                 if (declaration.kind === SyntaxKind.PropertyAccessExpression) {
                     // Declarations only exist for property access expressions for certain

tests/cases/fourslash/getJavaScriptSemanticDiagnostics24.ts

@@ -0,0 +1,16 @@
+/// <reference path="fourslash.ts" />
+
+// @allowJs: true
+// @Filename: a.js
+//// function Person(age) {
+////     if (age >= 18) {
+////         this.canVote = true;
+////     } else {
+////         this.canVote = 23;
+////     }
+//// }
+//// let x = new Person(100);
+//// x.canVote/**/;
+
+goTo.marker();
+verify.quickInfoIs('(property) Person.canVote: boolean | number');