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

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 0fda4e48c5fdea78e20feddb11eb607bde8ba93b..684f3eb746f3d71fd6b4f8c58ac6e65a1875ee73 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -6598,10 +6598,46 @@ RawString* Function::QualifiedUserVisibleName() const {
 }
 
 
-RawString* Function::GetSource() {
+RawString* Function::GetSource() const {
+  if (IsImplicitConstructor() || IsSignatureFunction()) {
+    // We may need to handle more cases when the restrictions on mixins are
+    // relaxed. In particular we might start associating some source with the
+    // forwarding constructors when it becomes possible to specify a particular
+    // constructor from the mixin to use.
+    return String::null();
+  }
   const Script& func_script = Script::Handle(script());
-  // Without the + 1 the final "}" is not included.
-  return func_script.GetSnippet(token_pos(), end_token_pos() + 1);
+  const TokenStream& stream = TokenStream::Handle(func_script.tokens());
+  if (!func_script.HasSource()) {
+    // When source is not available, avoid printing the whole token stream and
+    // doing expensive position calculations.
+    return stream.GenerateSource(token_pos(), end_token_pos() + 1);
+  }
+
+  const TokenStream::Iterator tkit(stream, end_token_pos());
+  intptr_t from_line;
+  intptr_t from_col;
+  intptr_t to_line;
+  intptr_t to_col;
+  func_script.GetTokenLocation(token_pos(), &from_line, &from_col);
+  func_script.GetTokenLocation(end_token_pos(), &to_line, &to_col);
+  intptr_t last_tok_len = String::Handle(tkit.CurrentLiteral()).Length();
+  // Handle special cases for end tokens of closures (where we exclude the last
+  // token):
+  // (1) "foo(() => null, bar);": End token is `,', but we don't print it.
+  // (2) "foo(() => null);": End token is ')`, but we don't print it.
+  // (3) "var foo = () => null;": End token is `;', but in this case the token
+  // semicolon belongs to the assignment so we skip it.
+  if ((tkit.CurrentTokenKind() == Token::kCOMMA) ||                   // Case 1.
+      (tkit.CurrentTokenKind() == Token::kRPAREN) ||                  // Case 2.
+      (tkit.CurrentTokenKind() == Token::kSEMICOLON &&
+       String::Handle(name()).Equals("<anonymous closure>"))) {  // Case 3.
+    last_tok_len = 0;
+  }
+  const String& result = String::Handle(func_script.GetSnippet(
+      from_line, from_col, to_line, to_col + last_tok_len));
+  ASSERT(!result.IsNull());
+  return result.raw();
 }
 
 
@@ -8401,16 +8437,6 @@ RawString* Script::GetLine(intptr_t line_number) const {
 }
 
 
-RawString* Script::GetSnippet(intptr_t from_token_pos,
-                              intptr_t to_token_pos) const {
-  intptr_t from_line, from_column;
-  intptr_t to_line, to_column;
-  GetTokenLocation(from_token_pos, &from_line, &from_column);
-  GetTokenLocation(to_token_pos, &to_line, &to_column);
-  return GetSnippet(from_line, from_column, to_line, to_column);
-}
-
-
 RawString* Script::GetSnippet(intptr_t from_line,
                               intptr_t from_column,
                               intptr_t to_line,