Extract ClosedWorldImpl from WorldImpl.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index fc92457466981e395bde54e00941a30cef90ba5d..2861f6b8fe352731fd64603ca007ef67ccb48548 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -47,7 +47,7 @@ class CompilerImpl extends Compiler {
 
   CompilerImpl(this.provider, api.CompilerOutput outputProvider, this.handler,
       CompilerOptions options,
-      {MakeBackendFuncion makeBackend, MakeReporterFunction makeReporter})
+      {MakeBackendFunction makeBackend, MakeReporterFunction makeReporter})
       // NOTE: allocating measurer is done upfront to ensure the wallclock is
       // started before other computations.
       : measurer = new Measurer(enableTaskMeasurements: options.verbose),