Start implementing checked mode and tools support for using it.

lib/compiler/implementation/ssa/codegen_helpers.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.
 
 /**
  * Instead of emitting each SSA instruction with a temporary variable
  * mark instructions that can be emitted at their use-site.
  * For example, in:
  *   t0 = 4;
  *   t1 = 3;
@@ skipping 24 matching lines @@
     for (HInstruction input in instruction.inputs) {
       if (!generateAtUseSite.contains(input)
           && !input.isCodeMotionInvariant()
           && input.usedBy.length == 1
           && input is! HPhi) {
         expectedInputs.add(input);
       }
     }
   }
 
-  void visitTypeConversion(HTypeConversion instruction) {
-    generateAtUseSite.add(instruction);
-  }
-
   // The codegen might use the input multiple times, so it must not be
   // set generate at use site.
   void visitIs(HIs instruction) {}
 
   // A check method must not have its input generate at use site,
   // because it's using it multiple times.
   void visitCheck(HCheck instruction) {}
 
   // A type guard should not generate its input at use site, otherwise
   // they would not be alive.
@@ skipping 433 matching lines @@
     // improving the performance of future lookups.
     T root = getRepresentative(parent);
     if (root !== parent) representative[element] = root;
     return root;
   }
 
   bool areEquivalent(T a, T b) {
     return getRepresentative(a) === getRepresentative(b);
   }
 }