Re-apply "Clean up the patch file for the isolate library by introducing a new buitlin library: iso…

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(revision 16320)
+++ sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart	(working copy)
@@ -1913,9 +1913,9 @@
     if (compiler.isMockCompilation) return;
     Element main = compiler.mainApp.find(Compiler.MAIN);
     String mainCall = null;
-    if (compiler.isolateLibrary != null) {
+    if (compiler.hasIsolateSupport()) {
       Element isolateMain =
-        compiler.isolateLibrary.find(Compiler.START_ROOT_ISOLATE);
+        compiler.isolateHelperLibrary.find(Compiler.START_ROOT_ISOLATE);
       mainCall = buildIsolateSetup(buffer, main, isolateMain);
     } else {
       mainCall = '${namer.isolateAccess(main)}()';