"extents" instead of "extends" generates a large number of confusing error messages in the analyzer

[Hixie]

The following code has a one-character typo:

class A {
  A(this.foo);
  final int foo;
}

class B extents A {
  B(int foo) : super(foo);
  
  int bar() {
    return foo;
  }
}

...and triggers at least 15 errors in the analyzer.