Implement type cast in the VM.

runtime/vm/token.h

Index: runtime/vm/token.h
===================================================================
--- runtime/vm/token.h	(revision 9048)
+++ runtime/vm/token.h	(working copy)
@@ -14,7 +14,7 @@
 //  13  multiplicative  * / ~/ %
 //  12  additive        + -
 //  11  shift           << >>
-//  10  relational      >= > <= < is
+//  10  relational      >= > <= < is as
 //   9  equality        == != === !==
 //   8  bitwise and     &
 //   7  bitwise xor     ^
@@ -106,8 +106,11 @@
   TOK(kGTE, ">=", 10, kNoAttribute)                                            \
                                                                                \
   /* Internal token for !(expr is Type) negative type test operator */         \
-  TOK(kISNOT, "", 0, kNoAttribute)                                             \
+  TOK(kISNOT, "", 10, kNoAttribute)                                            \
                                                                                \
+  /* Internal token for (expr as Type) type cast operator */                   \
+  TOK(kAS, "", 10, kNoAttribute)                                               \
+                                                                               \
   TOK(kINDEX, "[]", 0, kNoAttribute)                                           \
   TOK(kASSIGN_INDEX, "[]=", 0, kNoAttribute)                                   \
                                                                                \
@@ -212,10 +215,14 @@
     return kEQ <= tok && tok <= kNE_STRICT;
   }
 
-  static bool IsInstanceofOperator(Kind tok) {
+  static bool IsTypeTestOperator(Kind tok) {
     return (tok == kIS) || (tok == kISNOT);
   }
 
+  static bool IsTypeCastOperator(Kind tok) {
+    return tok == kAS;
+  }
+
   static bool IsPseudoKeyword(Kind tok) {
     return (Attributes(tok) & kPseudoKeyword) != 0;
   }