Make StringBuffer and interpolation behave the same.

dart/sdk/lib/core/string_buffer.dart

Index: dart/sdk/lib/core/string_buffer.dart
diff --git a/dart/sdk/lib/core/string_buffer.dart b/dart/sdk/lib/core/string_buffer.dart
index 3c517ea86be339d847b6ed71b7ec5ad17a60c50f..89b0d212e70511730b09bdd81e3c53da0ea78f2b 100644
--- a/dart/sdk/lib/core/string_buffer.dart
+++ b/dart/sdk/lib/core/string_buffer.dart
@@ -54,8 +54,8 @@ class _StringBufferImpl implements StringBuffer {
 
   /// Adds [obj] to the buffer.
   void add(Object obj) {
-    String str = obj.toString();
-    if (str == null || str.isEmpty) return;
+    String str = '$obj';

[srdjan, 2012/12/20 16:40:29]

For the VM, string interpolation is slower than calling toString and I do not
see benefits. if 'obj' does not have a toString method that returns String, than
we throw an illegal argument error when interpolating. We can to the same
explicitly here.

[Ivan Posva, 2012/12/20 16:42:41]

I do not see the purpose of this change. Please explain this particular line.

[ahe, 2012/12/20 17:10:37]

The purpose of this change is to get sane semantics. Once we agree on the
semantics, both teams can optimize this function.  I have a CL with a faster
version for dart2js and will move this class to a VM specific patch file.

As far as I can tell, the current version will be harder for you to optimize.
The problem is the surprising behavior that happens if str is a non-empty list.

[sra1, 2012/12/20 19:07:41]

obj.toString() should almost never be called explicitly.
Interpolation is clearer at the source level and the guarantees it provides
gives more robust applications.

If
  '$obj'
is slower than
  temp = obj.toString();
  if (temp is! String) throw ...
then fix the VM.

'$obj' is cleaner and guarantees a string.

Not only is preferring interpolation better style and better practice, it is
also better for compiled code size.  If, in the core library, we have a piece of
code that does a check, and another piece of code that does the same check, that
makes every application compiled to JavaScript bigger.  Presumably a VM snapshot
is a little bigger too.

[ahe, 2012/12/21 11:23:38]

I have changed the code to use an explicit check as I have a follow-up CL that
moves this code to a VM-specific patch file.

I think the VM team understands that string interpolation needs to be fast, and
my change represents what is, not what should be.

+    if (str.isEmpty) return;
     _buffer.add(str);
     _length += str.length;
   }