Cascades off by a line in DDC source maps

On the throw, the following shows the line with `bar` in the trace instead of `foo`.

```dart
class A {
  void foo(int t) {
    throw new Exception('Called foo with $t');
  }

  void bar(String s) {
    print('hello $s');
  }
}

void main() {
  new A()
    ..bar('world')
    ..foo(42);
}
```

@jmesserly 