oauth2/google: add config type to use Cloud SDK credentials

Change-Id: Ied7fecc0cb155c33faca7766b81221eacb3aa0c0
Reviewed-on: https://go-review.googlesource.com/1670
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Burcu Dogan <jbd@google.com>

google/sdk_test.go

@@ -0,0 +1,39 @@
+// Copyright 2015 The oauth2 Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package google
+
+import "testing"
+
+func TestSDKConfig(t *testing.T) {
+	unixHomeDir = "testdata"
+	tests := []struct {
+		account     string
+		accessToken string
+		err         bool
+	}{
+		{"", "bar_access_token", false},
+		{"foo@example.com", "foo_access_token", false},
+		{"bar@example.com", "bar_access_token", false},
+	}
+	for _, tt := range tests {
+		c, err := NewSDKConfig(tt.account)
+		if (err != nil) != tt.err {
+			if !tt.err {
+				t.Errorf("expected no error, got error: %v", tt.err, err)
+			} else {
+				t.Errorf("execcted error, got none")
+			}
+			continue
+		}
+		tok := c.initialToken
+		if tok == nil {
+			t.Errorf("expected token %q, got: nil", tt.accessToken)
+			continue
+		}
+		if tok.AccessToken != tt.accessToken {
+			t.Errorf("expected token %q, got: %q", tt.accessToken, tok.AccessToken)
+		}
+	}
+}