Remove some duplication in code that extracts the source for a function.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 12fc18d330ac0e76cb5a751d42bbbf876d6a0d20..58590858a74526bde3a9123e4479cfe90364cf6f 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1709,7 +1709,7 @@ class Function : public Object {
   RawString* QualifiedUserVisibleName() const;
   virtual RawString* DictionaryName() const { return name(); }
 
-  RawString* GetSource();
+  RawString* GetSource() const;
 
   // Build a string of the form 'C<T, R>(T, {b: B, c: C}) => R' representing the
   // internal signature of the given function. In this example, T and R are
@@ -2790,8 +2790,6 @@ class Script : public Object {
 
   RawLibrary* FindLibrary() const;
   RawString* GetLine(intptr_t line_number) const;
-  RawString* GetSnippet(intptr_t from_token_pos,
-                        intptr_t to_token_pos) const;
   RawString* GetSnippet(intptr_t from_line,
                         intptr_t from_column,
                         intptr_t to_line,