Port over collect like terms.
Port over collect like terms.

Test Plan:
http://localhost:3000/?start=x+2x
http://localhost:3000/?start=2x+2x
http://localhost:3000/?start=2xy+2xy
Select everything see that it collects correctly.

Khan/algebra-tool,kevinbarabash/mobile-mathipulator,Khan/algebra-tool,kevinbarabash/mobile-mathipulator