feat(TestBed): add whitespace handling to compile

Add whitespace handling to the TestBed’s compile function.

Closes #1262. Previously, strings passed with leading or
trailing whitespace would be compiled incorrectly. Now,
valid HTML with arbitrary whitespace in and surrounding it
can be compiled properly.
```
return html.split('\n')
           .map((line) {
             var trimmed = line.trim();
             return trimmed + (trimmed.isEmpty || trimmed.endsWith('>') ? '' : ' '); })
           .join();
```