Allow dart2dart to output one file per library.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
index b4c2001b31b6478030e1b3dc516b09feddf8b25e..4ae02a0623da974312402a111a70adbf31b87c67 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
@@ -25,6 +25,9 @@ class DartBackend extends Backend {
   final bool outputAst = false;
   final Map<ClassNode, List<Node>> memberNodes;
 
+  /// If `true`, libraries are generated into separate files.
+  final bool multiFile;
+
   PlaceholderRenamer placeholderRenamer;
 
   // TODO(zarah) Maybe change this to a command-line option.
@@ -102,7 +105,7 @@ class DartBackend extends Backend {
     return true;
   }
 
-  DartBackend(Compiler compiler, List<String> strips)
+  DartBackend(Compiler compiler, List<String> strips, {this.multiFile})
       : tasks = <CompilerTask>[],
         memberNodes = new Map<ClassNode, List<Node>>(),
         forceStripTypes = strips.indexOf('types') != -1,
@@ -264,6 +267,9 @@ class DartBackend extends Backend {
       }
     }
 
+    List<LibraryElement> userLibraries =
+        compiler.libraryLoader.libraries.where(isUserLibrary).toList();
+
     Set<Element> topLevelElements = new Set<Element>();
     Map<ClassElement, Set<Element>> classMembers =
         new Map<ClassElement, Set<Element>>();
@@ -452,18 +458,76 @@ class DartBackend extends Backend {
                                topLevelNodes, collector);
     }
 
-    final EmitterUnparser unparser =
-        new EmitterUnparser(placeholderRenamer.renames,
-                            stripTypes: forceStripTypes,
-                            minify: compiler.enableMinification);
-    for (LibraryElement library in placeholderRenamer.platformImports) {
-      if (library.isPlatformLibrary && !library.isInternalLibrary) {
-        unparser.unparseImportTag(library.canonicalUri.toString());
+    Map<LibraryElement, String> outputPaths = new Map<LibraryElement, String>();
+    Map<LibraryElement, EmitterUnparser> unparsers =
+        new Map<LibraryElement, EmitterUnparser>();
+
+    // The single unparser used if we collect all the output in one file
+    EmitterUnparser mainUnparser = multiFile
+        ? null
+        : new EmitterUnparser(placeholderRenamer.renames,
+            stripTypes: forceStripTypes,
+            minify: compiler.enableMinification);
+
+    if (multiFile) {
+      String mainName = compiler.outputUri.pathSegments.last;
+      String mainBaseName = mainName.endsWith(".dart")
+          ? mainName.substring(0, mainName.length - 5)
+          : mainName;
+      // Map each library to a path based on the uri of the original
+      // library and [compiler.outputUri].
+      Set<String> usedLibraryPaths = new Set<String>();
+      for (LibraryElement library in userLibraries) {
+        if (library == compiler.mainApp) {
+          outputPaths[library] = mainBaseName;
+        } else {
+          List<String> names =
+              library.canonicalUri.pathSegments.last.split(".");
+          if (names.last == "dart") {
+            names = names.sublist(0, names.length - 1);
+          }
+          outputPaths[library] =
+              "$mainBaseName.${makeUnique(names.join("."), usedLibraryPaths)}";
+        }
+      }
+
+      for(LibraryElement outputLibrary in userLibraries) {

[floitsch, 2014/08/28 20:10:07]

Add comment explaining that you rewrite the import tags here.

Move into a different function?

[sigurdm, 2014/09/03 08:24:16]

Done.
Added a TODO

+        EmitterUnparser unparser = new EmitterUnparser(
+            placeholderRenamer.renames,
+            stripTypes: forceStripTypes,
+            minify: compiler.enableMinification);
+        unparsers[outputLibrary] = unparser;
+        LibraryName libraryName = outputLibrary.libraryTag;
+        if (libraryName != null) {
+          unparser.visitLibraryName(libraryName);
+        }
+        for (LibraryTag tag in outputLibrary.tags) {
+          if (tag is! LibraryDependency) continue;
+          LibraryDependency dependency = tag;
+          LibraryElement libraryElement =
+              outputLibrary.getLibraryFromTag(dependency);
+          String uri = outputPaths.containsKey(libraryElement)
+              ? "${outputPaths[libraryElement]}.dart"
+              : libraryElement.canonicalUri.toString();
+          if (dependency is Import) {
+            unparser.unparseImportTag(uri);
+          } else {
+            unparser.unparseExportTag(uri);
+          }
+        }
+      }
+    } else {
+      for(LibraryElement library in placeholderRenamer.platformImports) {
+        if (library.isPlatformLibrary && !library.isInternalLibrary) {
+          mainUnparser.unparseImportTag(library.canonicalUri.toString());
+        }
       }
     }
+
     for (int i = 0; i < sortedTopLevels.length; i++) {
       Element element = sortedTopLevels[i];
       Node node = topLevelNodes[i];
+      Unparser unparser = multiFile ? unparsers[element.library] : mainUnparser;
       if (node is ClassNode) {
         // TODO(smok): Filter out default constructors here.
         unparser.unparseClassWithBody(node, memberNodes[node]);
@@ -473,13 +537,27 @@ class DartBackend extends Backend {
       unparser.newline();
     }
 
-    compiler.assembledCode = unparser.result;
-    compiler.outputProvider("", "dart")
-         ..add(compiler.assembledCode)
-         ..close();
-    // Output verbose info about size ratio of resulting bundle to all
-    // referenced non-platform sources.
-    logResultBundleSizeInfo(topLevelElements);
+    if (multiFile) {
+      for(LibraryElement outputLibrary in userLibraries) {
+        compiler.outputProvider(outputPaths[outputLibrary], "dart")
+    // TODO(sigurdm): Make the unparser output directly into the buffer instead

[floitsch, 2014/08/28 20:10:06]

Write comment before statement.

[sigurdm, 2014/09/03 08:24:16]

Done.

+    // of caching in `.result`.
+             ..add(unparsers[outputLibrary].result)
+             ..close();
+      }
+      // TODO(sigurdm): What to do here? Probably we should get rid of

[floitsch, 2014/08/28 20:10:06]

Agreed.
Until then we usually write the main unit into that string.

[sigurdm, 2014/09/03 08:24:16]

Done.

+      // compiler.assembledCode
+      compiler.assembledCode = "";
+    } else {
+      compiler.assembledCode = mainUnparser.result;
+      compiler.outputProvider("", "dart")
+           ..add(compiler.assembledCode)
+           ..close();
+
+      // Output verbose info about size ratio of resulting bundle to all
+      // referenced non-platform sources.
+      logResultBundleSizeInfo(topLevelElements);

[floitsch, 2014/08/28 20:10:07]

Can we do something similar?

[sigurdm, 2014/09/03 08:24:16]

Done.

+    }
   }
 
   void logResultBundleSizeInfo(Set<Element> topLevelElements) {