Add support for the package: directive in Frog and dart2js.

lib/compiler/implementation/apiimpl.dart

Index: lib/compiler/implementation/apiimpl.dart
diff --git a/lib/compiler/implementation/apiimpl.dart b/lib/compiler/implementation/apiimpl.dart
index 4301708c40a05ee726362258b91490a18f9a907f..78cf17d41c0323dcb519979c2d39223c11d5e26b 100644
--- a/lib/compiler/implementation/apiimpl.dart
+++ b/lib/compiler/implementation/apiimpl.dart
@@ -19,6 +19,7 @@ class Compiler extends leg.Compiler {
   ReadUriFromString provider;
   DiagnosticHandler handler;
   Uri libraryRoot;
+  Uri packageRoot;
   List<String> options;
   bool mockableLibraryUsed = false;
 
@@ -38,6 +39,8 @@ class Compiler extends leg.Compiler {
   leg.Script readScript(Uri uri, [leg.ScriptTag node]) {
     if (uri.scheme == 'dart') {
       uri = translateDartUri(uri, node);
+    } else if (uri.scheme == 'package') {
+      uri = translatePackageUri(uri, node);
     }
     String text = "";
     try {
@@ -65,7 +68,11 @@ class Compiler extends leg.Compiler {
     return libraryRoot.resolve(path);
   }
 
+  translatePackageUri(Uri uri, leg.ScriptTag node) =>

[Bob Nystrom, 2012/04/09 23:54:08]

I <3 => too, but I think a regular {} body is better when it wraps to another
line.

[nweiz, 2012/04/10 00:16:31]

I think it's fine for a single-line body, even if that body's on the next line.
This pattern is pretty common in the existing codebase. We should have John
declare it one way or the other in the style guide.

+    packageRoot.resolve(uri.path);
+
   bool run(Uri uri) {
+    packageRoot = uri.resolve('packages/');
     bool success = super.run(uri);
     for (final task in tasks) {
       log('${task.name} took ${task.timing}msec');