Begin implementing subtype matching for type inference.

pkg/front_end/lib/src/fasta/type_inference/type_schema.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
index 5268f62d08a7fe41c855b3e687cfe6c78730b768..8342a2be9388e9794a3e65aeef280761407291a8 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema.dart
@@ -6,6 +6,9 @@ import 'package:kernel/ast.dart';
 import 'package:kernel/import_table.dart';
 import 'package:kernel/text/ast_to_text.dart';
 
+/// Determines whether a type schema contains `?` somewhere inside it.
+bool isKnown(DartType schema) => schema.accept(new _IsKnownVisitor());

[Jennifer Messerly, 2017/05/04 18:06:01]

since the visitor is stateless, would it be worth a lazy static/top-level field
so we aren't allocating a new one each time?

e.g.

class _IsKnownVisitor {
  ...
  static final instance = new _IsKnownVisitor();
  ...
}


... IDK whether this codebase is doing this sort of optimization at this point,
but it's a pretty easy one as they go :)

[Paul Berry, 2017/05/04 19:56:07]

Actually what I'd really love to do is change this to

bool isKnown(DartType schema) => schema.accept(const _IsKnownVisitor());

(that way the compiler double checks that the visitor is stateless)

I'll do that in a follow-up CL.

+
 /// Converts a [DartType] to a string, representing the unknown type as `?`.
 String typeSchemaToString(DartType schema) {
   StringBuffer buffer = new StringBuffer();
@@ -73,3 +76,40 @@ class UnknownType extends DartType {
   @override
   visitChildren(Visitor v) {}
 }
+
+/// Visitor that computes [isKnown].
+class _IsKnownVisitor extends TypeSchemaVisitor<bool> {
+  @override
+  bool defaultDartType(DartType node) => true;
+
+  @override
+  bool visitFunctionType(FunctionType node) {
+    if (!node.returnType.accept(this)) return false;
+    for (var parameterType in node.positionalParameters) {
+      if (!parameterType.accept(this)) return false;
+    }
+    for (var namedParameterType in node.namedParameters) {
+      if (!namedParameterType.type.accept(this)) return false;
+    }
+    return true;
+  }
+
+  @override
+  bool visitInterfaceType(InterfaceType node) {
+    for (var typeArgument in node.typeArguments) {
+      if (!typeArgument.accept(this)) return false;
+    }
+    return true;
+  }
+
+  @override
+  bool visitTypedefType(TypedefType node) {
+    for (var typeArgument in node.typeArguments) {
+      if (!typeArgument.accept(this)) return false;
+    }
+    return true;
+  }
+
+  @override
+  bool visitUnknownType(UnknownType node) => false;
+}