Add support for the --js and --externs flags to the GWT version

Closes #3036

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=207211332

src/com/google/javascript/jscomp/CommandLineRunner.java

@@ -857,8 +857,6 @@ private void parse(List<String> args) throws CmdLineException {
 
     private static final ImmutableSet<String> gwtUnsupportedFlags =
         ImmutableSet.of(
-            "externs",
-            "js",
             "conformance_configs",
             "error_format",
             "warnings_whitelist_file",

src/com/google/javascript/jscomp/gwt/client/GwtRunner.java

@@ -104,13 +104,15 @@ private static class Flags {
     String[] entryPoint;
     String env;
     boolean exportLocalPropertyDefinitions;
+    Object[] externs;
     String[] extraAnnotationName;
     String[] forceInjectLibraries;
     String[] formatting;
     boolean generateExports;
     String[] hideWarningsFor;
     boolean injectLibraries;
     String isolationMode;
+    String[] js;
     String[] jscompError;
     String[] jscompOff;
     String[] jscompWarning;
@@ -142,7 +144,6 @@ private static class Flags {
     // These flags do not match the Java compiler JAR.
     @Deprecated
     File[] jsCode;
-    File[] externs;
     JsMap defines;
   }
 
@@ -186,6 +187,7 @@ private static Flags getDefaultFlags() {
     defaultFlags.generateExports = false;
     defaultFlags.hideWarningsFor = null;
     defaultFlags.injectLibraries = true;
+    defaultFlags.js = null;
     defaultFlags.jsCode = null;
     defaultFlags.jscompError = null;
     defaultFlags.jscompOff = null;
@@ -283,6 +285,28 @@ private native Object get(String key) /*-{
     }-*/;
   }
 
+  /**
+   * @param jsFilePaths Array of file paths. If running under NodeJS, they will be loaded via the
+   *     native node fs module.
+   * @return Array of File objects. If called without running under node, return null to indicate
+   *     failure.
+   */
+  private static native File[] getJsFiles(String[] jsFilePaths) /*-{
+    if (!(typeof process === 'object' && process.version)) {
+      return null;
+    }
+    var jsFiles = [];
+    for (var i = 0; i < jsFilePaths.length; i++) {
+      if (typeof process === 'object' && process.version) {
+        jsFiles.push({
+          path: jsFilePaths[i],
+          src: require('fs').readFileSync(jsFilePaths[i], 'utf8')
+        });
+      }
+    }
+    return jsFiles;
+  }-*/;
+
   @JsMethod(name = "keys", namespace = "Object")
   private static native String[] keys(Object o);
 
@@ -315,7 +339,7 @@ private static ChunkOutput writeChunkOutput(
         parseModuleWrappers(Arrays.asList(getStringArray(flags, "chunkWrapper")), chunks);
 
     for (JSModule c : chunks) {
-      if (flags.createSourceMap != null) {
+      if (flags.createSourceMap != null && !flags.createSourceMap.equals(false)) {
         compiler.getSourceMap().reset();
       }
 
@@ -363,7 +387,7 @@ private static ChunkOutput writeChunkOutput(
 
       file.src = out.toString();
 
-      if (flags.createSourceMap != null) {
+      if (flags.createSourceMap != null && !flags.createSourceMap.equals(false)) {
         StringBuilder b = new StringBuilder();
         try {
           compiler.getSourceMap().appendTo(b, file.path);
@@ -413,7 +437,7 @@ private static ChunkOutput writeOutput(Compiler compiler, Flags flags) {
       }
       postfix = flags.outputWrapper.substring(pos + marker.length());
     }
-    if (flags.createSourceMap != null) {
+    if (flags.createSourceMap != null && !flags.createSourceMap.equals(false)) {
       StringBuilder b = new StringBuilder();
       try {
         compiler.getSourceMap().appendTo(b, flags.jsOutputFile);
@@ -671,6 +695,29 @@ private static void applyOptionsFromFlags(
     options.setModuleRoots(Arrays.asList(getStringArray(flags, "jsModuleRoot")));
   }
 
+  /**
+   * @param externs Array of strings or File[]. If running under NodeJS, an array of strings will be
+   *     treated as file paths and loaded via the native node fs module.
+   * @return Array of extern File objects. If an array of strings is passed without running under
+   *     node, return null to indicate failure.
+   */
+  private static native File[] fromExternsFlag(Object[] externs) /*-{
+    var externFiles = [];
+    for (var i = 0; i < externs.length; i++) {
+      if (externs[i].path || externs[i].src) {
+        externFiles.push(externs[i]);
+      } else if (typeof process === 'object' && process.version) {
+        externFiles.push({
+          path: externs[i],
+          src: require('fs').readFileSync(externs[i], 'utf8')
+        });
+      } else {
+        return null;
+      }
+    }
+    return externFiles;
+  }-*/;
+
   private static List<SourceFile> fromFileArray(File[] src, String unknownPrefix) {
     List<SourceFile> out = new ArrayList<>();
     if (src != null) {
@@ -769,12 +816,30 @@ public static ChunkOutput compile(Flags flags, File[] inputs) throws IOException
       }
     }
 
+    if (flags.js != null) {
+      File[] jsFiles = getJsFiles(getStringArray(flags, "js"));
+      if (jsFiles == null) {
+        throw new RuntimeException(
+            "Can only load files from the filesystem when running in NodeJS.");
+      } else {
+        jsCode.addAll(fromFileArray(jsFiles, "Input_"));
+      }
+    }
+
     Compiler compiler = new Compiler(new NodePrintStream());
     CompilerOptions options = new CompilerOptions();
     applyOptionsFromFlags(options, flags, compiler.getDiagnosticGroups());
     options.setInputSourceMaps(sourceMaps);
 
-    List<SourceFile> externs = fromFileArray(flags.externs, "Extern_");
+    List<SourceFile> externs = new ArrayList<>();
+    if (flags.externs != null) {
+      File[] externFiles = fromExternsFlag(getStringArray(flags, "externs"));
+      if (externFiles == null) {
+        throw new RuntimeException(
+            "Can only load files from the filesystem when running in NodeJS.");
+      }
+      externs = fromFileArray(externFiles, "Extern_");
+    }
     externs.addAll(createExterns(options.getEnvironment()));
 
     NodeErrorManager errorManager = new NodeErrorManager();