Recognize "patch" identifier in .dartp files.

lib/compiler/implementation/lib/math.dartp

Index: lib/compiler/implementation/lib/math.dartp
diff --git a/lib/compiler/implementation/lib/math.dartp b/lib/compiler/implementation/lib/math.dartp
index 2901563f4f5857f7beaff73530f1d104dbcbec79..af061bff0c7e2bee778958b74b8a8715f8d2213d 100644
--- a/lib/compiler/implementation/lib/math.dartp
+++ b/lib/compiler/implementation/lib/math.dartp
@@ -4,9 +4,10 @@
 
 // Patch file for dart:math library.
 
-// Imports the MathNatives class used by the library.
+// Imports the MathNatives class used by the library as well as checkNum used
+// below.
 #import("js_helper.dart");
 
 // Override the sqrt function, as proof-of-concept.
-double sqrt(num value)
+patch double sqrt(num value)
   => JS('double', @'Math.sqrt(#)', checkNum(value));