Use interpolation instead of concatenation.

LayoutTests/dart/fib.dart

 class Main {
   static int fib(int n) {
     if (n < 2)
       return n;
     else
       return fib(n - 1) + fib(n - 2);
   }
 
   // FIXME: Make this a unit test.
   static HTMLBodyElement get body() { return document.body; }
 
   static void main() {
-    log("fib(10) = " + fib(10));
+    log("fib(10) = ${fib(10)}");
     if (fib(10) != 55) {
         log("FAIL");
         return;
     }
     log("PASS");
   }
 
   static void log(String msg) {
     HTMLElement element = document.createElement('div');
     element.innerHTML = msg;
     body.appendChild(element);
   }
 }