go/types: fix Eval to use correct file set when evaluating an expression

This is https://go-review.googlesource.com/10999 which we could not apply
in x/tools/go/types because we must not rely on 1.5 features in that repo
yet.

Change-Id: I9a57cdb7ad4051df278d1fbed90c736df50f426f
Reviewed-on: https://go-review.googlesource.com/11125Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/eval.go

@@ -65,11 +65,7 @@ func Eval(fset *token.FileSet, pkg *Package, pos token.Pos, expr string) (tv Typ
 	}
 
 	// parse expressions
-	// BUG(gri) In case of type-checking errors below, the type checker
-	//          doesn't have the correct file set for expr. The correct
-	//          solution requires a ParseExpr that uses the incoming
-	//          file set fset.
-	node, err := parser.ParseExpr(expr)
+	node, err := parser.ParseExprFrom(fset, "eval", expr, 0)
 	if err != nil {
 		return TypeAndValue{}, err
 	}