cps-ir: Implement string concatenation.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index d97abb45d0812d06cdc18b90137c920ec581bbec..0755833ef5afd14e3d92b05b3256744f195182b4 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -134,8 +134,28 @@ class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
 
   @override
   js.Expression visitConcatenateStrings(tree_ir.ConcatenateStrings node) {
-    return giveup(node);
-    // TODO: implement visitConcatenateStrings
+    js.Expression addStrings(js.Expression left, js.Expression right) {
+      return new js.Binary('+', left, right);
+    }
+
+    js.Expression toString(tree_ir.Expression input) {
+      bool useDirectly = input is tree_ir.Constant &&
+          (input.expression.value.isString ||
+           input.expression.value.isInt ||
+           input.expression.value.isBool);
+      js.Expression value = visit(input);
+      if (useDirectly) {
+        return value;
+      } else {
+        Element convertToString = glue.getStringConversion();
+        registry.registerStaticUse(convertToString);
+        js.Expression access = glue.staticFunctionAccess(convertToString);
+        return (new js.Call(access, <js.Expression>[value]));
+      }
+    }
+
+    Iterable<js.Expression> parts = node.arguments.map(toString);
+    return parts.skip(1).fold(parts.first, addStrings);

[asgerf, 2015/02/06 10:37:25]

Could be: parts.reduce(addStrings)

   }
 
   @override