Add tests of line numbers to causal async stack tests

tests/language/vm/causal_async_exception_stack_test.dart

Index: tests/language/vm/causal_async_exception_stack_test.dart
diff --git a/tests/language/vm/causal_async_exception_stack_test.dart b/tests/language/vm/causal_async_exception_stack_test.dart
index ed4147fd2760cd16265ec4ebd8e6776188db5c9f..ee59bfb68b9a6d07ffc4c449a818a6fd64c122b9 100644
--- a/tests/language/vm/causal_async_exception_stack_test.dart
+++ b/tests/language/vm/causal_async_exception_stack_test.dart
@@ -29,11 +29,11 @@ main() async {
     await foo();
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([
-        'thrower',
+        'thrower', ':8:',
         '<asynchronous suspension>',
-        'generator',
+        'generator', ':17:',
         '<asynchronous suspension>',
-        'foo',
+        'foo', ':21:',
         '<asynchronous suspension>',
         'main',
         ]));
@@ -67,9 +67,9 @@ main() async {
     await thrower();
   } catch(e, st) {
     expect(st.toString(), stringContainsInOrder([
-           'thrower',
+           'thrower', ':8:',
            '<asynchronous suspension>',
-           'main',
+           'main', ':67:',
            ]));
   }
 }