build: Try to get and use working directory, when possible.

Previously, in build.Import, the srcDir argument to importWithSrcDir
call was always empty string.

Ideally, the Import method should have a srcDir parameter and pass it
onwards. Then the caller (gopherjs tool, for example) can provide the
working directory, if available. However, this is an exported method
and changing its signature at this time is hard. Settle for an
intermediate solution of try to figure out working directory inside
Import, but continue to use empty string on failure (such as if in
GOARCH=js mode, e.g., inside browser).

This will be used by the gopherjs tool in the next commit to be able to
resolve relative import paths, such as "." or "./foo/bar".

The future plan is to bring all the "supported" code into gopherjs repo
for convenience, mark the build API as unstable and make breaking API
changes to bring it up to speed.

Helps #302.

build/build.go

@@ -62,7 +62,14 @@ func NewBuildContext(installSuffix string, buildTags []string) *build.Context {
 // If an error occurs, Import returns a non-nil error and a nil
 // *PackageData.
 func Import(path string, mode build.ImportMode, installSuffix string, buildTags []string) (*PackageData, error) {
-	return importWithSrcDir(path, "", mode, installSuffix, buildTags)
+	wd, err := os.Getwd()
+	if err != nil {
+		// Getwd may fail if we're in GOARCH=js mode. That's okay, handle
+		// it by falling back to empty working directory. It just means
+		// Import will not be able to resolve relative import paths.
+		wd = ""
+	}
+	return importWithSrcDir(path, wd, mode, installSuffix, buildTags)
 }
 
 func importWithSrcDir(path string, srcDir string, mode build.ImportMode, installSuffix string, buildTags []string) (*PackageData, error) {