Re-apply 'Unify dart:isolate.' with fixes for autogenerated dart2js and dartium html files.

lib/isolate/base.dart

Index: lib/isolate/base.dart
diff --git a/lib/isolate/isolate_api.dart b/lib/isolate/base.dart
similarity index 96%
rename from lib/isolate/isolate_api.dart
rename to lib/isolate/base.dart
index 010007e2b5bd63f8ae59a3420ec226f2593fa35c..b5d92bfcd43e6c4df66ac5eb4e55c641f6c44b5f 100644
--- a/lib/isolate/isolate_api.dart
+++ b/lib/isolate/base.dart
@@ -14,7 +14,7 @@ class IsolateSpawnException implements Exception {
  * the first communication between isolates (see [spawnFunction] and
  * [spawnUri]).
  */
-ReceivePort get port() => _port;
+external ReceivePort get port();
 
 /**
  * Creates and spawns an isolate that shares the same code as the current
@@ -32,9 +32,7 @@ ReceivePort get port() => _port;
  * See comments at the top of this library for more details.
  */
 // Note this feature is not yet available in the dartvm.
-SendPort spawnFunction(void topLevelFunction()) {
-  return _spawnFunction(topLevelFunction);
-}
+external SendPort spawnFunction(void topLevelFunction());
 
 /**
  * Creates and spawns an isolate whose code is available at [uri].  Like with
@@ -43,9 +41,7 @@ SendPort spawnFunction(void topLevelFunction()) {
  *
  * See comments at the top of this library for more details.
  */
-SendPort spawnUri(String uri) {
-  return _spawnUri(uri);
-}
+external SendPort spawnUri(String uri);
 
 /**
  * [SendPort]s are created from [ReceivePort]s. Any message sent through
@@ -146,3 +142,7 @@ interface SendPortSync {
   callSync(var message);
 
 }
+
+class _ReceivePortFactory {
+  external factory ReceivePort();
+}