Fix compilation errors

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 8d3c9a275ee0b59f6333d51a4969ef8b8e1e700d..6f9d81c8142b2e57e67c5c36be1d519ea15f6d29 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -843,7 +843,7 @@ class CodeChecker extends RecursiveAstVisitor {
       } else {
         // Future<T> -> FutureOr<T>
         var typeArg = (type.element == typeProvider.futureType.element)
-            ? type.typeArguments[0]
+            ? (type as InterfaceType).typeArguments[0]
             : typeProvider.dynamicType;
         return typeProvider.futureOrType.instantiate([typeArg]);
       }
@@ -930,7 +930,7 @@ class CodeChecker extends RecursiveAstVisitor {
     // Special case for FutureOr to handle returned values from async functions.
     // In this case, we're more permissive than assignability.
     if (to.element == typeProvider.futureOrType.element) {
-      var to1 = to.typeArguments[0];
+      var to1 = (to as InterfaceType).typeArguments[0];
       var to2 = typeProvider.futureType.instantiate([to1]);
       return _needsImplicitCast(expr, to1, from: from) == true ||
           _needsImplicitCast(expr, to2, from: from) == true;