Add an option to print the source for a single file after each pass.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132083348

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

@@ -966,7 +966,7 @@ final void beforePass(String passName) {
   @Override
   final void afterPass(String passName) {
     if (options.printSourceAfterEachPass) {
-      String currentJsSource = toSource();
+      String currentJsSource = getCurrentJsSource();
       if (!currentJsSource.equals(this.lastJsSource)) {
         System.out.println();
         System.out.println("// " + passName + " yields:");
@@ -977,6 +977,25 @@ final void afterPass(String passName) {
     }
   }
 
+  final String getCurrentJsSource() {
+    String filename = options.fileToPrintAfterEachPass;
+    if (filename == null) {
+      return toSource();
+    } else {
+      Node script = getScriptNode(filename);
+      return script != null ? toSource(script) : ("File '" + filename + "' not found");
+    }
+  }
+
+  final Node getScriptNode(String filename) {
+    for (Node file : jsRoot.children()) {
+      if (file.getSourceFileName() != null && file.getSourceFileName().endsWith(filename)) {
+        return file;
+      }
+    }
+    return null;
+  }
+
   /**
    * Returns a new tracer for the given pass name.
    */

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

@@ -880,15 +880,21 @@ public void setTrustedStrings(boolean yes) {
     trustedStrings = yes;
   }
 
-  String reportPath;
-
-  // Should only be used when debugging compiler bugs using small JS inputs.
+  // Should only be used when debugging compiler bugs.
   boolean printSourceAfterEachPass;
+  // Used to narrow down the printed source when overall input size is large.
+  String fileToPrintAfterEachPass;
 
   public void setPrintSourceAfterEachPass(boolean printSource) {
     this.printSourceAfterEachPass = printSource;
   }
 
+  public void setFileToPrintAfterEachPass(String filename) {
+    this.fileToPrintAfterEachPass = filename;
+  }
+
+  String reportPath;
+
   /** Where to save a report of global name usage */
   public void setReportPath(String reportPath) {
     this.reportPath = reportPath;