Store canonical type argments in a hash table instead of linear list.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 15913)
+++ runtime/vm/object.h	(working copy)
@@ -919,6 +919,8 @@
   virtual bool IsInstantiated() const;
   virtual bool IsUninstantiatedIdentity() const;
 
+  virtual intptr_t Hash() const;
+
  private:
   // Check if this type argument vector consists solely of DynamicType,
   // considering only a prefix of length 'len'.
@@ -3249,6 +3251,8 @@
     return BuildName(kUserVisibleName);
   }
 
+  virtual intptr_t Hash() const;
+
   // The name of this type's class, i.e. without the type argument names of this
   // type.
   RawString* ClassName() const;
@@ -3361,6 +3365,8 @@
       const AbstractTypeArguments& instantiator_type_arguments) const;
   virtual RawAbstractType* Canonicalize() const;
 
+  virtual intptr_t Hash() const;
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawType));
   }
@@ -3459,6 +3465,8 @@
       const AbstractTypeArguments& instantiator_type_arguments) const;
   virtual RawAbstractType* Canonicalize() const { return raw(); }
 
+  virtual intptr_t Hash() const;
+
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawTypeParameter));
   }