Fixes for checked mode and change buildbot script to run dart2js tests in checked mode also.

lib/compiler/implementation/ssa/builder.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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 290 matching lines @@
   void startFunction(FunctionElement function,
                      FunctionExpression node) {
 
     ClosureTranslator translator = new ClosureTranslator(builder);
     closureData = translator.translate(node);
 
     FunctionSignature params = function.computeSignature(builder.compiler);
     params.forEachParameter((Element element) {
       HInstruction parameter = new HParameterValue(element);
       builder.add(parameter);
-      parameter = builder.potentiallyCheckType(parameter, element);
+      builder.potentiallyCheckType(parameter, element);
       directLocals[element] = parameter;
     });
 
     enterScope(node);
 
     // If the freeVariableMapping is not empty, then this function was a
     // nested closure that captures variables. Redirect the captured
     // variables to fields in the closure.
     closureData.freeVariableMapping.forEach((Element from, Element to) {
       redirectElement(from, to);
@@ skipping 3108 matching lines @@
   void visitNodeList(NodeList node) {
      node.visitChildren(this);
   }
 
   HInstruction concat(HInstruction left, HInstruction right) {
     HInstruction instruction = new HStringConcat(left, right, node);
     builder.add(instruction);
     return instruction;
   }
 }