Start work on a non-complete type inferrer. Currently only analyzes return types.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 1513 matching lines @@
     if (!user.hasSameLoopHeaderAs(node)) return;
 
     // Replace the user with a [HOneShotInterceptor].
     HConstant nullConstant = graph.addConstantNull(constantSystem);
     List<HInstruction> inputs = new List<HInstruction>.from(user.inputs);
     inputs[0] = nullConstant;
     HOneShotInterceptor interceptor = new HOneShotInterceptor(
         user.selector, inputs, node.interceptedClasses);
     interceptor.sourcePosition = user.sourcePosition;
     interceptor.sourceElement = user.sourceElement;
+    interceptor.guaranteedType = user.guaranteedType;
 
     HBasicBlock block = user.block;
     block.addAfter(user, interceptor);
     block.rewrite(user, interceptor);
     block.remove(user);
 
     // The interceptor will be removed in the dead code elimination
     // phase. Note that removing it here would not work because of how
     // the [visitBasicBlock] is implemented.
   }
 }