internal/analysisinternal: disable AddImport test without go command

The AddImport test uses the default importer, which calls go list.
This fails in environments that can't call the go command, like
js/wasm.

Add a predicate to testenv that asserts the need for the default
importer, and call it from TestAddImport.

A subtlety: although this bug manifested itself only for the dot-import
cases, in fact all the test cases failed type checking on js/wasm for
this reason. But a successful type-check is not a precondition for
the test (see the new comment in TestAddImport). What caused the
particular test case to fail was a bad diff resulting from how the edit
was applied in the presence of that failure.

Fixes golang/go#71645.

Change-Id: Ib04afad108c323999bb67f329cf8d9cf329fead1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/648580
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: jba

internal/analysisinternal/addimport_test.go

@@ -18,9 +18,12 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/internal/analysisinternal"
+	"golang.org/x/tools/internal/testenv"
 )
 
 func TestAddImport(t *testing.T) {
+	testenv.NeedsDefaultImporter(t)
+
 	descr := func(s string) string {
 		if _, _, line, ok := runtime.Caller(1); ok {
 			return fmt.Sprintf("L%d %s", line, s)
@@ -270,6 +273,9 @@ func _(io.Reader) {
 				Implicits: make(map[ast.Node]types.Object),
 			}
 			conf := &types.Config{
+				// We don't want to fail if there is an error during type checking:
+				// the error may be because we're missing an import, and adding imports
+				// is the whole point of AddImport.
 				Error:    func(err error) { t.Log(err) },
 				Importer: importer.Default(),
 			}

internal/testenv/testenv.go

@@ -278,6 +278,16 @@ func NeedsGoBuild(t testing.TB) {
 	NeedsTool(t, "go")
 }
 
+// NeedsDefaultImporter skips t if the test uses the default importer,
+// returned by [go/importer.Default].
+func NeedsDefaultImporter(t testing.TB) {
+	t.Helper()
+	// The default importer may call `go list`
+	// (in src/internal/exportdata/exportdata.go:lookupGorootExport),
+	// so check for the go tool.
+	NeedsTool(t, "go")
+}
+
 // ExitIfSmallMachine emits a helpful diagnostic and calls os.Exit(0) if the
 // current machine is a builder known to have scarce resources.
 //