fix #30427, casting tearoffs to the wrong type

pkg/analyzer/lib/src/task/strong/checker.dart

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index 568a59856d7493306e0db16ce98b1e570b9bb27a..e09f6af3f3b8fec50d6c82c46e312c3ac9826b7d 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -913,13 +913,20 @@ class CodeChecker extends RecursiveAstVisitor {
       var expectedType = member.returnType;

[Jennifer Messerly, 2017/08/11 22:06:07]

If we wanted to fix other covariant tearoffs by casting at tearoff point, we
could do it here.

instead of using ".returnType" on both sides, just evaluate the full type:


var worstCaseType = _lookUpMember(classLowerBound, member).type;
var expectedType = member.type

var isMethod = member is MethodElement;
var isCall = node is MethodInvocation;
var isTearOff = isMethod && !isCall;
if (!isTearOff) {
  // For non-tearoff, we're only interested in the return type.
  expectedType = expectedType.returnType;
  worstCaseType = worstCaseType.returnType;
}

... rest of code as normal & revert the changes I'm making below, because
tearoffs won't be special anymore and "expectedType" will mean the right thing
:)

 
       if (!rules.isSubtypeOf(memberLowerBound.returnType, expectedType)) {
-        if (node is MethodInvocation && member is! MethodElement) {
-          // If `o.m` is not a method, we need to cast `o.m` before the call:
-          // `(o.m as expectedType)(args)`.
+        var isMethod = member is MethodElement;
+        var isCall = node is MethodInvocation;
+
+        if (isMethod && !isCall) {
+          // If `o.m` is a method tearoff, cast to the method type.
+          setImplicitCast(node, member.type);
+        } else if (!isMethod && isCall) {
+          // If `o.g()` is calling a field/getter `g`, we need to cast `o.g`
+          // before the call: `(o.g as expectedType)(args)`.
           // This cannot be represented by an `as` node without changing the
           // Dart AST structure, so we record it as a special cast.
           setImplicitOperationCast(node, expectedType);
         } else {
+          // For method calls `o.m()` or getters `o.g`, simply cast the result.
           setImplicitCast(node, expectedType);
         }
         _hasImplicitCasts = true;