Implement a list tracer phase, that tries to find element types in individual lists.

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/compiler.dart	(revision 24023)
+++ sdk/lib/_internal/compiler/implementation/compiler.dart	(working copy)
@@ -405,6 +405,7 @@
   EnqueueTask enqueuer;
   CompilerTask fileReadingTask;
   DeferredLoadTask deferredLoadTask;
+  ContainerTracer containerTracer;
   String buildId;
 
   static const SourceString MAIN = const SourceString('main');
@@ -506,6 +507,7 @@
       closureToClassMapper = new closureMapping.ClosureTask(this, closureNamer),
       checker = new TypeCheckerTask(this),
       typesTask = new ti.TypesTask(this),
+      containerTracer = new ContainerTracer(this),
       constantHandler = new ConstantHandler(this, backend.constantSystem),
       deferredLoadTask = new DeferredLoadTask(this),
       enqueuer = new EnqueueTask(this)];