Support type variables on redirecting factory constructors.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 1ea7a23e8d21d4eccf6346b944dabd76a0547fcc..d1ad31e07858a4a55278d2958e883af5b7e65910 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -317,6 +317,7 @@ class ErroneousElementX extends ElementX implements ErroneousElement {
   get origin => unsupported();
   get defaultImplementation => unsupported();
 
+  bool get isRedirectingFactory => unsupported();
   bool get isPatched => unsupported();
   bool get isPatch => unsupported();
 
@@ -1093,6 +1094,38 @@ class FunctionSignatureX implements FunctionSignature {
   }
 
   int get parameterCount => requiredParameterCount + optionalParameterCount;
+
+  /**
+   * Check whether a function with this signature can be used instead of a
+   * function with signature [signature] without causing a `noSuchMethod`
+   * exception/call.
+   */
+  bool isCompatibleWith(FunctionSignature signature) {
+    if (optionalParametersAreNamed != signature.optionalParametersAreNamed) {
+      return false;
+    }
+    if (optionalParametersAreNamed) {
+      if (requiredParameterCount != signature.requiredParameterCount) {
+        return false;
+      }
+      for (Element namedParameter in signature.optionalParameters) {
+        if (!optionalParameters.contains(namedParameter)) {
+          return false;
+        }
+      }
+    } else {
+      // There must be at least as many arguments as in the other signature, but
+      // this signature must not have more required paramters.  Having more
+      // optional parameters is not a problem, they simply are never provided
+      // by call sites of a call to a method with the other signature.
+      if (requiredParameterCount > signature.requiredParameterCount ||
+          requiredParameterCount < signature.parameterCount ||
+          parameterCount < signature.parameterCount) {
+        return false;
+      }
+    }
+    return true;
+  }
 }
 
 class FunctionElementX extends ElementX implements FunctionElement {
@@ -1152,6 +1185,8 @@ class FunctionElementX extends ElementX implements FunctionElement {
   bool get isPatched => patch != null;
   bool get isPatch => origin != null;
 
+  bool get isRedirectingFactory => defaultImplementation != this;
+
   FunctionElement get redirectionTarget {
     if (this == defaultImplementation) return this;
     var target = defaultImplementation;