Fix auto-generated externs conflicting with closure externs

For every `@JS.Import`, Web Image generates an externs definition for
that type. If it is a JS built-in type like `Map`, the closure compiler
will also have an externs definition for it, which results in a multiple
declarations error.

We suppress those warnings in our generated externs and put the closure
externs first so that the generated ones comes after and the suppression
actually works

Author: patrick96

web-image/src/com.oracle.svm.hosted.webimage.closurecompiler/src/com/oracle/svm/hosted/webimage/closurecompiler/ClosureCompilerSupportImpl.java

@@ -109,32 +109,44 @@ public String applyClosureCompiler(String inputSourceCode) {
 
         final CompilerOptions closureCompOpts = createClosureCompilerOptions();
 
-        List<SourceFile> externs = new ArrayList<>();
+        /*
+         * Collect externs we generate in separate list so that only those are dumped when
+         * pre-closure dumping is turned on.
+         */
+        List<SourceFile> ourExterns = new ArrayList<>();
 
         // Add externs file that exposes the user-facing Web Image VM API.
-        externs.add(vmApiExterns());
+        ourExterns.add(vmApiExterns());
         // Add externs file for externally defined JavaScript classes.
-        externs.add(jsExterns());
+        ourExterns.add(jsExterns());
 
         /*
          * Node.js externs are always needed due to the feature detection referencing node-specific
          * variables
          */
-        externs.add(nodejsExterns());
+        ourExterns.add(nodejsExterns());
 
         // Dump all externs files generated by Web Image
-        for (SourceFile f : externs) {
+        for (SourceFile f : ourExterns) {
             dumpPreClosure(f.getName(), f::getCode);
         }
 
+        List<SourceFile> externs = new ArrayList<>();
+        /*
+         * Add the closure compiler externs first. Otherwise, those externs may redeclare symbols in
+         * our externs and fail the compilation. Our externs suppress those warnings to not fail the
+         * compilation.
+         */
         try {
             // TODO also add externs for stuff we use in our code (e.g process.argv for node)
             // With that we could enable property renaming
             externs.addAll(AbstractCommandLineRunner.getBuiltinExterns(closureCompOpts.getEnvironment()));
         } catch (IOException e) {
-            VMError.shouldNotReachHere(e);
+            throw VMError.shouldNotReachHere(e);
         }
 
+        externs.addAll(ourExterns);
+
         final Compiler closureCompiler = new Compiler();
         Result r = compile(closureCompiler, externs, sourceCode, closureCompOpts);
         if (!r.warnings.isEmpty()) {
@@ -338,14 +350,16 @@ private SourceFile jsExterns() {
         for (Map.Entry<String, JSCodeGenTool.ExternClassDescriptor> entry : variables.entrySet()) {
             String var = entry.getKey();
             JSCodeGenTool.ExternClassDescriptor descriptor = entry.getValue();
-            sb.append("var ").append(var).append(" = " + getExternClass(descriptor) + ";\n");
+            sb.append("/** @suppress {checkVars|duplicate} */\n");
+            sb.append("var ").append(var).append(" = ").append(getExternClass(descriptor)).append(";\n");
             sb.append("\n");
         }
 
         for (Map.Entry<String, JSCodeGenTool.ExternClassDescriptor> entry : components.entrySet()) {
             String comp = entry.getKey();
             JSCodeGenTool.ExternClassDescriptor descriptor = entry.getValue();
-            sb.append(comp).append(" = " + getExternClass(descriptor) + ";\n");
+            sb.append("/** @suppress {checkVars|duplicate} */\n");
+            sb.append(comp).append(" = ").append(getExternClass(descriptor)).append(";\n");
             sb.append("\n");
         }