Canonicalize all integer constants taht we create by parsing a string.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 15418)
+++ runtime/vm/object.h	(working copy)
@@ -3464,7 +3464,9 @@
 
 class Integer : public Number {
  public:
-  static RawInteger* New(const String& str, Heap::Space space = Heap::kNew);
+  // Returns a canonical Integer object allocated in the old gen space.
+  static RawInteger* NewCanonical(const String& str);
+
   static RawInteger* New(int64_t value, Heap::Space space = Heap::kNew);
 
   virtual double AsDoubleValue() const;
@@ -3621,7 +3623,8 @@
     return RoundedAllocationSize(sizeof(RawBigint) + (len * kBytesPerElement));
   }
 
-  static RawBigint* New(const String& str, Heap::Space space = Heap::kNew);
+  // Returns a canonical Bigint object allocated in the old gen space.
+  static RawBigint* NewCanonical(const String& str);
 
   RawBigint* ArithmeticOp(Token::Kind operation, const Bigint& other) const;