Check boolean conversion in checked mode.

lib/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.
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 562 matching lines @@
     return node;
   }
 
   HInstruction visitTypeConversion(HTypeConversion node) {
     HInstruction value = node.inputs[0];
     DartType type = types[node].computeType(compiler);
     if (type.element === compiler.dynamicClass
         || type.element === compiler.objectClass) {
       return value;
     }
+    if (types[value].canBeNull() && node.isBooleanConversionCheck) {
+      return node;
+    }
     HType combinedType = types[value].intersection(types[node]);
     return (combinedType == types[value]) ? value : node;
   }
 
   Element findConcreteFieldForDynamicAccess(HInstruction receiver,
                                             Selector selector) {
     HType receiverType = types[receiver];
     if (!receiverType.isUseful()) return null;
     if (receiverType.canBeNull()) return null;
     DartType type = receiverType.computeType(compiler);
@@ skipping 773 matching lines @@
       // this type for the field is still a strong signal
       // indicating the expected type of the field.
       types[field] = type;
     } else {
       // If there are no invoked setters we know the type of
       // this field for sure.
       field.guaranteedType = type;
     }
   }
 }