add the --legacy-peer-deps config

docs/content/using-npm/config.md

@@ -644,6 +644,25 @@ such as the one included with node 0.8, can install the package.  This
 eliminates all automatic deduping. If used with `global-style` this option
 will be preferred.
 
+#### legacy-peer-deps
+
+* Default: false
+* Type: Boolean
+
+Causes npm to completely ignore `peerDependencies` when building a package
+tree, as in npm versions 3 through 6.
+
+If a package cannot be installed because of overly strict
+`peerDependencies` that collide, it provides a way to move forward
+resolving the situation.
+
+This differs from `--omit=peer`, in that `--omit=peer` will avoid unpacking
+`peerDependencies` on disk, but will still design a tree such that
+`peerDependencies` _could_ be unpacked in a correct place.
+
+Use of `legacy-peer-deps` is not recommended, as it will not enforce the
+`peerDependencies` contract that meta-dependencies may rely on.
+
 #### link
 
 * Default: false

lib/config/defaults.js

@@ -171,6 +171,7 @@ Object.defineProperty(exports, 'defaults', {get: function () {
     json: false,
     key: null,
     'legacy-bundling': false,
+    'legacy-peer-deps': false,
     link: false,
     'local-address': undefined,
     loglevel: 'notice',
@@ -316,6 +317,7 @@ exports.types = {
   json: Boolean,
   key: [null, String],
   'legacy-bundling': Boolean,
+  'legacy-peer-deps': Boolean,
   link: Boolean,
   'local-address': getLocalAddresses(),
   loglevel: ['silent', 'error', 'warn', 'notice', 'http', 'timing', 'info', 'verbose', 'silly'],

lib/config/flat-options.js

@@ -165,6 +165,7 @@ const flatOptions = npm => npm.flatOptions || Object.freeze({
   legacyBundling: npm.config.get('legacy-bundling'),
   scriptShell: npm.config.get('script-shell') || undefined,
   omit: buildOmitList(npm),
+  legacyPeerDeps: npm.config.get('legacy-peer-deps'),
 
   // used to build up the appropriate {add:{...}} options to Arborist.reify
   save: npm.config.get('save'),