go/ast/astutil: fix panic in DeleteNamedImport from line directive

DeleteNamedImport used FileSet.Position() where it should have
used FileSet.PositionFor(, false) to ignore line directives.

There are likely many more of these panics lurking in x/tools.

Change-Id: I11fc0ea5301fe873cb679a82e231b1720dbdaef6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/398596
Run-TryBot: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Trust: Peter Weinberger <pjw@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

go/ast/astutil/imports.go

@@ -270,8 +270,8 @@ func DeleteNamedImport(fset *token.FileSet, f *ast.File, name, path string) (del
 			}
 			if j > 0 {
 				lastImpspec := gen.Specs[j-1].(*ast.ImportSpec)
-				lastLine := fset.Position(lastImpspec.Path.ValuePos).Line
-				line := fset.Position(impspec.Path.ValuePos).Line
+				lastLine := fset.PositionFor(lastImpspec.Path.ValuePos, false).Line
+				line := fset.PositionFor(impspec.Path.ValuePos, false).Line
 
 				// We deleted an entry but now there may be
 				// a blank line-sized hole where the import was.

go/ast/astutil/imports_test.go

@@ -1654,6 +1654,34 @@ import f "fmt"
 `,
 		unchanged: true,
 	},
+	// this test panics without PositionFor in DeleteNamedImport
+	{
+		name:       "import.44",
+		pkg:        "foo.com/other/v3",
+		renamedPkg: "",
+		in: `package main
+//line mah.go:600
+
+import (
+"foo.com/a.thing"
+"foo.com/surprise"
+"foo.com/v1"
+"foo.com/other/v2"
+"foo.com/other/v3"
+)
+`,
+		out: `package main
+
+//line mah.go:600
+
+import (
+	"foo.com/a.thing"
+	"foo.com/other/v2"
+	"foo.com/surprise"
+	"foo.com/v1"
+)
+`,
+	},
 }
 
 func TestDeleteImport(t *testing.T) {