Support checked mode for return type.

lib/compiler/implementation/ssa/optimize.dart

Index: lib/compiler/implementation/ssa/optimize.dart
===================================================================
--- lib/compiler/implementation/ssa/optimize.dart	(revision 14460)
+++ lib/compiler/implementation/ssa/optimize.dart	(working copy)
@@ -398,7 +398,7 @@
     assert(!leftType.isConflicting() && !rightType.isConflicting());
 
     // We don't optimize on numbers to preserve the runtime semantics.
-    if (!(left.isNumber(types) && right.isNumber(types)) &&
+    if (!(left.isNumberOrNull(types) && right.isNumberOrNull(types)) &&
         leftType.intersection(rightType, compiler).isConflicting()) {
       return graph.addConstantBool(false, constantSystem);
     }
@@ -620,7 +620,9 @@
     HInstruction value = node.inputs[1];
     if (compiler.enableTypeAssertions) {
       HInstruction other = value.convertType(
-          compiler, field, HTypeConversion.CHECKED_MODE_CHECK);
+          compiler,
+          field.computeType(compiler),
+          HTypeConversion.CHECKED_MODE_CHECK);
       if (other != value) {
         node.block.addBefore(node, other);
         value = other;