Convert String to a class.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index e17b5dd615e299041c85734ba3ac793d568fc1ee..eeb8989c86ea4e260b5fb1f3120813e85830c24d 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3185,8 +3185,8 @@ class AbstractType : public Instance {
   // Check if this type represents the 'num' type.
   bool IsNumberType() const;
 
-  // Check if this type represents the 'String' interface.
-  bool IsStringInterface() const;
+  // Check if this type represents the 'String' type.
+  bool IsStringType() const;
 
   // Check if this type represents the 'Function' type.
   bool IsFunctionType() const;
@@ -3309,8 +3309,8 @@ class Type : public AbstractType {
   // The 'num' interface type.
   static RawType* Number();
 
-  // The 'String' interface type.
-  static RawType* StringInterface();
+  // The 'String' type.
+  static RawType* StringType();
 
   // The 'Function' interface type.
   static RawType* Function();