Fix the duplicate function implementation error that depended on order of files

Author: sheetalkamat

src/compiler/checker.ts

@@ -10770,6 +10770,7 @@ namespace ts {
 
             let symbol = getSymbolOfNode(node);
             let firstDeclaration = getDeclarationOfKind(symbol, node.kind);
+
             // Only type check the symbol once
             if (node === firstDeclaration) {
                 checkFunctionOrConstructorSymbol(symbol);
@@ -11782,7 +11783,13 @@ namespace ts {
                 let symbol = getSymbolOfNode(node);
                 let localSymbol = node.localSymbol || symbol;
 
-                let firstDeclaration = getDeclarationOfKind(localSymbol, node.kind);
+                let firstDeclaration = forEach(symbol.declarations, declaration => {
+                    // Get first non javascript function declaration
+                    if (declaration.kind === node.kind && !isJavaScript(getSourceFile(declaration).fileName)) {
+                        return declaration;
+                    }
+                });
+
                 // Only type check the symbol once
                 if (node === firstDeclaration) {
                     checkFunctionOrConstructorSymbol(localSymbol);

tests/cases/fourslash/jsFileCompilationDuplicateFunctionImplementation.ts

@@ -11,10 +11,14 @@
 // @emitThisFile: true
 ////function foo() { return 30; }/*2*/
 
+goTo.marker("1");
+verify.getSemanticDiagnostics('[]');
 goTo.marker("2");
-verify.getSemanticDiagnostics("[]");
+verify.getSemanticDiagnostics('[\n  {\n    "message": "Duplicate function implementation.",\n    "start": 9,\n    "length": 3,\n    "category": "error",\n    "code": 2393\n  }\n]');
 verify.verifyGetEmitOutputContentsForCurrentFile([
     { fileName: "out.js", content: "function foo() { return 10; }\r\nfunction foo() { return 30; }\r\n" },
     { fileName: "out.d.ts", content: "" }]);
 goTo.marker("2");
-verify.getSemanticDiagnostics('[\n  {\n    "message": "Duplicate function implementation.",\n    "start": 9,\n    "length": 3,\n    "category": "error",\n    "code": 2393\n  }\n]');
+verify.getSemanticDiagnostics('[\n  {\n    "message": "Duplicate function implementation.",\n    "start": 9,\n    "length": 3,\n    "category": "error",\n    "code": 2393\n  }\n]');
+goTo.marker("1");
+verify.getSemanticDiagnostics('[]');