Missing type information in expression compilation can cause valid expression to be compiled incorrectly

[sjindel-google]

Currently the API for expression compilation in the front end is provided a set of Strings representing
the variables in scope. CFE gives all these variables the type dynamic. This can lead to some types being incorrectly inferred:

Source code:

class D<T> {
  Y id<Y>(Y x) => x;
  m(List<T> l);
  foo() {
    List<T> s;
    // breakpoint here
  }
}

Expression:

m(id(s = []))

Since the type of s is seen as dynamic, CFE will output:

method /* from org-dartlang-debug:synthetic_debug_expression */ debugExpr(dynamic s) → dynamic
  return this.{main::D::m}(this.{main::D::id}(let dynamic _ = null in s = <dynamic>[]));

This will raise a runtime type error when entering m, whereas if the user had actually written this expression at the breakpoint, it would run ok.

[rmacnak-google]

@aam @rmacnak-google

[kmillikin]

This is a tool usability issue, but not directly related to the Dart 2.1 release.