61457 Add assert to Table to check for rows with null children (cla: yes, framework)

61474 Fix Border.symmetric: phase 1 (cla: yes, framework, waiting for tree to go green)

61479 Update MaterialState API docs (cla: yes, f: material design, framework, waiting for tree to go green)

61485 fix intrinsic height and width for widget span (cla: yes, framework, waiting for tree to go green)

61489 Updated date pickers to use the new TextButton in place of FlatButton. (cla: yes, f: material design, framework, waiting for tree to go green)

61492 Updated time pickers to use the new TextButton in place of FlatButton. (cla: yes, f: material design, framework)