Add JSNull, JSBool and JSFunction, and move toString into the new interceptor scheme.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(revision 15139)
+++ sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(working copy)
@@ -443,6 +443,10 @@
         type = HType.INTEGER;
       } else if (cls == builder.backend.jsDoubleClass) {
         type = HType.DOUBLE;
+      } else if (cls == builder.backend.jsNullClass) {
+        type = HType.NULL;
+      } else if (cls == builder.backend.jsBoolClass) {
+        type = HType.BOOLEAN;
       }
       value.guaranteedType = type;
     }