dashboard: don't test the exp repo against release branches

Exp is experimental. It's hard enough keeping it passing on master.
We're not going to fight release branches as well. If somebody wants
to see test coverage against three versions of Go, they can graduate
their code out of experimental.

Change-Id: I4695ce479c47bd56a4a58dba05d367d3de020c58
Reviewed-on: https://go-review.googlesource.com/c/build/+/170518
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -1010,6 +1010,10 @@ func (c *BuildConfig) buildsRepoAtAll(repo, branch, goBranch string) bool {
 			if bmm.Less(c.MinimumGoVersion) {
 				return false
 			}
+			if repo == "exp" {
+				// Don't test exp against release branches; it's experimental.
+				return false
+			}
 		}
 	}
 

dashboard/builders_test.go

@@ -436,6 +436,11 @@ func TestBuilderConfig(t *testing.T) {
 		{b("darwin-386-10_11", "go"), onlyPost},
 		{b("darwin-386-10_11@go1.12", "go"), onlyPost},
 		{b("darwin-386-10_11@go1.11", "go"), onlyPost},
+
+		// exp is experimental; it only tests against master.
+		{b("linux-amd64", "exp"), both},
+		{b("linux-amd64@go1.11", "exp"), none},
+		{b("linux-amd64@go1.12", "exp"), none},
 	}
 	for _, tt := range tests {
 		t.Run(tt.br.testName, func(t *testing.T) {