Ugly. A pile of new xxxFlags() functions, to communicate to the parser

that 'yield' is a keyword.  This doesn't help test_generators at all!  I
don't know why not.  These things do work now (and didn't before this
patch):

1. "from __future__ import generators" now works in a native shell.

2. Similarly "python -i xxx.py" now has generators enabled in the
   shell if xxx.py had them enabled.

3. This program (which was my doctest proxy) works fine:

from __future__ import generators

source = """\
def f():
    yield 1
"""

exec compile(source, "", "single") in globals()
print type(f())

1 files changed, 30 insertions, 9 deletions

diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 0412b7e..1d20deb 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -533,6 +533,7 @@ PyRun_InteractiveOneFlags(FILE *fp, char *filename, PyCompilerFlags *flags)
 	node *n;
 	perrdetail err;
 	char *ps1 = "", *ps2 = "";
+
 	v = PySys_GetObject("ps1");
 	if (v != NULL) {
 		v = PyObject_Str(v);
@@ -549,8 +550,11 @@ PyRun_InteractiveOneFlags(FILE *fp, char *filename, PyCompilerFlags *flags)
 		else if (PyString_Check(w))
 			ps2 = PyString_AsString(w);
 	}
-	n = PyParser_ParseFile(fp, filename, &_PyParser_Grammar,
-			       Py_single_input, ps1, ps2, &err);
+	n = PyParser_ParseFileFlags(fp, filename, &_PyParser_Grammar,
+			    	    Py_single_input, ps1, ps2, &err,
+			    	    (flags &&
+			    	     flags->cf_flags & PyCF_GENERATORS) ?
+			    	    	PyPARSE_YIELD_IS_KEYWORD : 0);
 	Py_XDECREF(v);
 	Py_XDECREF(w);
 	if (n == NULL) {
@@ -1017,7 +1021,9 @@ PyObject *
 PyRun_FileExFlags(FILE *fp, char *filename, int start, PyObject *globals,
 		  PyObject *locals, int closeit, PyCompilerFlags *flags)
 {
-	node *n = PyParser_SimpleParseFile(fp, filename, start);
+	node *n = PyParser_SimpleParseFileFlags(fp, filename, start,
+			(flags && flags->cf_flags & PyCF_GENERATORS) ?
+				PyPARSE_YIELD_IS_KEYWORD : 0);
 	if (closeit)
 		fclose(fp);
 	return run_err_node(n, filename, globals, locals, flags);
@@ -1101,7 +1107,9 @@ Py_CompileStringFlags(char *str, char *filename, int start,
 {
 	node *n;
 	PyCodeObject *co;
-	n = PyParser_SimpleParseString(str, start);
+	n = PyParser_SimpleParseStringFlags(str, start,
+		(flags && flags->cf_flags & PyCF_GENERATORS) ?
+			PyPARSE_YIELD_IS_KEYWORD : 0);
 	if (n == NULL)
 		return NULL;
 	co = PyNode_CompileFlags(n, filename, flags);
@@ -1125,30 +1133,43 @@ Py_SymtableString(char *str, char *filename, int start)
 /* Simplified interface to parsefile -- return node or set exception */
 
 node *
-PyParser_SimpleParseFile(FILE *fp, char *filename, int start)
+PyParser_SimpleParseFileFlags(FILE *fp, char *filename, int start, int flags)
 {
 	node *n;
 	perrdetail err;
-	n = PyParser_ParseFile(fp, filename, &_PyParser_Grammar, start,
-				(char *)0, (char *)0, &err);
+	n = PyParser_ParseFileFlags(fp, filename, &_PyParser_Grammar, start,
+					(char *)0, (char *)0, &err, flags);
 	if (n == NULL)
 		err_input(&err);
 	return n;
 }
 
+node *
+PyParser_SimpleParseFile(FILE *fp, char *filename, int start)
+{
+	return PyParser_SimpleParseFileFlags(fp, filename, start, 0);
+}
+
 /* Simplified interface to parsestring -- return node or set exception */
 
 node *
-PyParser_SimpleParseString(char *str, int start)
+PyParser_SimpleParseStringFlags(char *str, int start, int flags)
 {
 	node *n;
 	perrdetail err;
-	n = PyParser_ParseString(str, &_PyParser_Grammar, start, &err);
+	n = PyParser_ParseStringFlags(str, &_PyParser_Grammar, start, &err,
+				      flags);
 	if (n == NULL)
 		err_input(&err);
 	return n;
 }
 
+node *
+PyParser_SimpleParseString(char *str, int start)
+{
+	return PyParser_SimpleParseStringFlags(str, start, 0);
+}
+
 /* Set the error appropriate to the given input error code (see errcode.h) */
 
 static void