Avoid mutating module export (#8775)

The `eth-contracts-metadata` export was mutated in `token-list.js` to
add the token address to the contract metadata, and this was taken
advantage of in the token search component.

Instead, the address is now added to a copy of the token metadata in
the one place it's used. The module export is no longer mutated, and
the unused `defaultTokens` variable in `token-list.js` has been
removed.

ui/app/components/app/token-list/token-list.js

@@ -1,6 +1,5 @@
 import React from 'react'
 import PropTypes from 'prop-types'
-import contracts from 'eth-contract-metadata'
 import { isEqual } from 'lodash'
 
 import TokenCell from '../token-cell'
@@ -10,15 +9,6 @@ import { useSelector } from 'react-redux'
 import { getAssetImages } from '../../../selectors'
 import { getTokens } from '../../../ducks/metamask/metamask'
 
-const defaultTokens = []
-for (const address in contracts) {
-  const contract = contracts[address]
-  if (contract.erc20) {
-    contract.address = address
-    defaultTokens.push(contract)
-  }
-}
-
 export default function TokenList ({ onTokenClick }) {
   const t = useI18nContext()
   const assetImages = useSelector(getAssetImages)

ui/app/pages/add-token/token-search/token-search.component.js

@@ -6,7 +6,7 @@ import InputAdornment from '@material-ui/core/InputAdornment'
 import TextField from '../../../components/ui/text-field'
 
 const contractList = Object.entries(contractMap)
-  .map(([ _, tokenData]) => tokenData)
+  .map(([address, tokenData]) => Object.assign({}, tokenData, { address }))
   .filter((tokenData) => Boolean(tokenData.erc20))
 
 const fuse = new Fuse(contractList, {