fix #470, string interpolation now respects Dart toString

lib/src/compiler/code_generator.dart

Index: lib/src/compiler/code_generator.dart
diff --git a/lib/src/compiler/code_generator.dart b/lib/src/compiler/code_generator.dart
index 05d603f8de8c428c91f543740d95d541006ad1c7..067c92d076146109c2968c2387076d176f3dfab5 100644
--- a/lib/src/compiler/code_generator.dart
+++ b/lib/src/compiler/code_generator.dart
@@ -4188,11 +4188,41 @@ class CodeGenerator extends GeneralizingAstVisitor
       _visitListToBinary(node.strings, '+');
 
   @override
-  JS.TemplateString visitStringInterpolation(StringInterpolation node) {
-    // Assuming we implement toString() on our objects, we can avoid calling it
-    // in most cases. Builtin types may differ though. We could handle this with
-    // a tagged template.
-    return new JS.TemplateString(_visitList(node.elements));
+  JS.Expression visitStringInterpolation(StringInterpolation node) {
+    var parts = [];
+    bool needsTag = false;
+    for (var interpolatePart in node.elements) {
+      if (!needsTag && interpolatePart is InterpolationExpression) {
+        // If we have something that could be native or null, we need to use
+        // dart.str
+        var expr = interpolatePart.expression;
+        var type = getStaticType(expr);
+
+        // JS template strings will call .toString() by default. This works for
+        // Dart types if we know they are not null or native. But if we have
+        // null, or a native type like List, we need to go through a runtime
+        // helper that will give the correct Dart behavior.
+        needsTag = isNullable(expr) ||
+            type.isDynamic ||
+            type.isObject ||
+            (_extensionTypes.hasNativeSubtype(type) &&
+                type != types.stringType);
+      }
+
+      parts.add(_visit(interpolatePart));
+    }
+
+    // Optimization: '$thing' is a common way to call toString.

[sra1, 2016/05/17 23:47:58]

It is not a synonym.

'$thing' will throw if toString() returns null.

dart2js has a helper for this case (called "S") that special-cases String and a
few other cases (e.g. number, but not zero which could be "0" or "-0.0") with JS
code.

[Jennifer Messerly, 2016/05/18 00:23:42]

Doh. If we want to support that then all readability improvements need to be
removed, since nothing can guarantee toString won't return null =(. A shame
because that has probably never happened in reality. Oh well, maybe we'll get
non-null-by-default types which will fix this at some point. =) For now I just
ripped out all of it.

+    if (parts.length == 3 && parts.first == '' && parts.last == '') {
+      var code = needsTag ? 'dart.toString(#)' : '#.toString()';
+      return js.call(code, parts[1]);
+    }
+
+    var templateStr = new JS.TemplateString(parts);
+    if (needsTag) {
+      return new JS.TaggedTemplate(js.call('dart.str'), templateStr);
+    }
+    return templateStr;
   }
 
   @override