Always enable evolving types in Javascript files

src/compiler/checker.ts

@@ -3239,10 +3239,10 @@ namespace ts {
                 return addOptionality(getTypeFromTypeNode(declaration.type), /*optional*/ declaration.questionToken && includeOptionality);
             }
 
-            if (compilerOptions.noImplicitAny &&
+            if ((compilerOptions.noImplicitAny || declaration.flags & NodeFlags.JavaScriptFile) &&
                 declaration.kind === SyntaxKind.VariableDeclaration && !isBindingPattern(declaration.name) &&
                 !(getCombinedModifierFlags(declaration) & ModifierFlags.Export) && !isInAmbientContext(declaration)) {
-                // If --noImplicitAny is on,
+                // If --noImplicitAny is on or the declaration is in a Javascript file,
                 // use control flow tracked 'any' type for non-ambient, non-exported var or let variables with no
                 // initializer or a 'null' or 'undefined' initializer.
                 if (!(getCombinedNodeFlags(declaration) & NodeFlags.Const) && (!declaration.initializer || isNullOrUndefined(declaration.initializer))) {