Change _interpolateSingle to use the same value in errors as _interpolate.

runtime/lib/string_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 const int _maxAscii = 0x7f;
 const int _maxLatin1 = 0xff;
 const int _maxUtf16 = 0xffff;
 const int _maxUnicode = 0x10ffff;
 
 patch class String {
@@ skipping 781 matching lines @@
       startIndex = match.end;
     }
     buffer.write(onNonMatch(this.substring(startIndex)));
     return buffer.toString();
   }
 
   // Convert single object to string.
   static String _interpolateSingle(Object o) {
     final s = o.toString();
     if (s is! String) {
-      throw new ArgumentError(o);
+      throw new ArgumentError(s);

[Lasse Reichstein Nielsen, 2015/02/12 13:11:09]

Using "o", which we already know has a faulty toString, means that the error
can't be printed either. We used "s" in _interpolate below.

Using "s" reduces the problem (but using Error.safeToString(s)) would likely be
even better.

[Ivan Posva, 2015/02/20 06:43:48]

Since you know s is not a String, you could also use o for the safeToString
call. I am not sure how much it helps here to try to use s in the ArgumentError
as s is not the faulty argument.

[sra1, 2015/02/24 16:57:02]

It seems a bad idea to report the wrong irritant like this (e.g. null).

We would not worry about this if ArgumentError was robust and reported a bad
object for the 'message' , e.g.  "Instance of ${o.runtimeType} (with bad
toString() method)", or "unprintable object" if that fails too.

That would also help other places that throw errors with bad toString methods.

We could still report something special here.

If this throw was changed to

   throw _interpolationNotStringError(o, s);

then the helper could be called from _interpolate too to ensure consistency,
probably throwing something like ArgumentError.value(o, 'interpolated value',
'toString() method does not return a String')

     }
     return s;
   }
 
   /**
    * Convert all objects in [values] to strings and concat them
    * into a result string.
    * Modifies the input list if it contains non-`String` values.
    */
   static String _interpolate(final List values) {
@@ skipping 480 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String input;
   final String pattern;
 }